The present invention relates to a magnetic head read/write preamplifier within a magnetic storage system. In particular, the present invention relates to a write driver circuit within the read/write preamplifier which is capable of applying a maximum voltage swing across a magnetic head in the magnetic storage system.
A magnetic storage system includes a magnetic head which reads and writes information on a recording surface of a magnetic media, such as a magnetic disk. The magnetic head includes an inductive coil which reads and writes the information by sensing or creating a changing magnetic field. A read/write preamplifier is connected to the magnetic head at first and second head contacts. The preamplifier includes read circuitry and write circuitry for controlling the read and write operations.
The write circuitry includes a write current driver circuit which is connected across the head contacts. During write mode operation, the write driver circuit forces a relatively large write current through the inductive coil to create a magnetic field that polarizes adjacent bit positions on the recording surface. Digital information is stored by reversing the polarization of selected bit positions which is done by reversing the direction of the current flow in the inductive coil.
A typical write driver circuit includes an "H-switch" for controlling the direction of current flow through the inductive coil. The H-switch includes upper write switching ("pull-up") transistors and lower write switching ("pull-down") transistors. The upper write switching transistors are connected between a first supply voltage and the head contacts. The lower write switching transistors are connected between the head contacts and a second supply voltage through a write current sink. The write current sink includes a write current control transistor connected in series with a resistor.
The write driver circuit controls the direction of current flow through the inductive coil by driving selected transistors in the H-switch between ON and OFF states. The write driver circuit applies a limited voltage swing across the head contacts for reversing current flow and polarizing the adjacent bit position.
The rate at which information can be stored on a recording surface through the magnetic head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The rise/fall time of the inductive coil is determined by: EQU di/dt-V/L 1
where di/dt is the rate of change of the current over time across the inductive coil, V is the available voltage across the inductive coil, and L is the load, which is an inductance. Therefore, the speed of the H-switch is directly proportional to the available voltage across the inductive coil.
The available voltage is determined by subtracting the voltage drops across the pull-up transistors, the pull-down transistors and the write current sink from the supply voltage. The available voltage is shown below: EQU V.sub.supply -[V.sub.be (pull-up)+V.sub.sat (pull-down)+V.sub.be (sink)+VR1(sink)] 2
Where:
V.sub.supply is the power supply voltage; PA1 V.sub.be (pull-up) is the turn on voltage at the pull-up transistor operating as an emitter follower, which is about 0.8 V; PA1 V.sub.sat (pull-down) is the saturation voltage drop across the pulldown transistor, which is about 0.4 V; PA1 V.sub.be (sink) is the turn on voltage of the write current control transistor in the write current sink, which is about 0.8 V; and PA1 VR1 is the voltage drop across the resistor in the current sink, which is about 0.4 V.
Therefore, the maximum voltage swing available across the inductive coil is: V.sub.supply -2.4 volts. For a preamplifier with a 5-volt supply, the available voltage swing is only 2.6 volts.
The available voltage at the load is increasingly significant in today's applications. As storage systems become more and more compact, there is a greater need for more compact voltage supplies. As a result, voltage capacity is often sacrificed to achieve a more compact voltage supply.
Portable computers are now available which operate on a 3.3 volt supply. With the conventional H-switch discussed above, the available voltage swing across the inductive coil in a 3.3 volt system is only 0.9 volts. Since switching speed is directly proportional to the available voltage swing, the use of a more compact 3.3 volt voltage supply results in a significant reduction in switching speed. A conventional H-switch supplies an inadequate amount of voltage at the load to store information effectively with a 3.3 volt power supply.
Non-synchronous switching is another problem with conventional H-switches. Conventional H-switches have two branches, each sending current through the load in a direction opposite the other branch. Non-synchronous switching between the two branches causes spiking and current overshoot at the load. This is especially a problem when fast NPN transistors are used to control the switching, as their speed makes them difficult to synchronize.
An additional problem with the conventional H-switch is the relatively large voltage swings generated at the head contacts during the write mode operation. Because current through the load (which is an inductor) cannot change instantaneously, the voltage swings have a tendency to rise above their forcing voltage causing a voltage spike. In the conventional H-switch this is particularly a problem because of the relatively low breakdown voltages of the switching transistors. The voltage spike occurs between the source and the load which corresponds to the base-emitter junction of the switching transistor. Typically these junctions breakdown at only 6 volts.
There is a continuing need to improve write driver circuits which increase the maximum voltage available at the load to improve overall switching speed and improve overall switching characteristics.